The present invention is a unipolar magneto-resistive preamplifier for use in a disc drive having a magneto-resistive element. Specifically, the preamplifier of the present invention utilizes a voltage regulator to maintain a voltage at one terminal of a magneto-resistive element, a first feedback loop to minimize a differential DC component and a differential low frequency component of the preamplifier output signal, and a second feedback loop to maintain a common mode component of the preamplifier output signals.
Magneto-resistive (MR) elements can read data from magnetic surfaces having data stored at high densities. In the disc drive art, MR elements are also referred to as MR heads or MR sensors. In addition, the term "magneto-restrictive" is sometimes used in place of the term "magneto-resistive."
The resistance of an MR element is, in part, a function of the strength of the magnetic field to which it is exposed. A read signal is generated by positioning the MR element proximate magnetic media, moving the magnetic media with respect to the element, and measuring the change in resistance of the element.
Typically, an MR element is biased by either a constant current or a constant voltage. As the magnetic flux passing through the element varies, a preamplifier detects the resistance changes of the MR element. In the constant current configuration, the change in voltage over the MR element is detected. In the constant voltage configuration, the change in current passing through the element is detected. The output of the preamplifier is then provided to other amplification and decoding circuitry for further processing.
Since the resistance of an MR element varies with magnetic field strength, the signal provided by the MR element is based not only on the data written on the magnetic media, but also on the distance between the element and the media. In addition, temperature and process variations also affect the steady-state resistance of an MR element. Accordingly, it is desirable to produce a preamplifier output signal that varies with changes in magnetic field strength, without varying significantly due to changes in steady-state resistance.
A preamplifier for producing such a signal was disclosed by Jove et al. in U.S. Pat. No. 4,706,138. Jove et al. disclosed a circuit wherein the amplified output signal represented the transient change in element resistance divided by the steady state resistance of the magnetic element, or alternatively, AR/R. The circuit disclosed by Jove et al. provided an improvement over the prior art because the output signal was not sensitive to resistance variations in individual MR elements and was not sensitive to the distance between the MR element and the surface of the magnetic media.
Another problem addressed in the prior art is protecting an MR element and a magnetic media surface from damage caused by a voltage discharge. Such a discharge can occur if a large enough voltage exists between the media surface and the element, and the two come sufficiently close to each other.
This problem is addressed by Jove et al., U.S. Pat. No. 4,879,610. In this patent, Jove et al. disclosed a protector circuit which maintains a fixed potential between the center of an MR element and an externally supplied reference voltage. A feedback loop employing a low pass filter is used to keep the voltage of the MR sensor at the same voltage as the externally applied reference voltage. The externally applied reference voltage is close to, if not the same as, the voltage at which the spindle and magnetic media are biased. In one embodiment, the spindle and media are connected to the disc drive's electrical ground, and the reference voltage is derived from a connection to the electrical ground. Accordingly, if the MR element should come into contact with the magnetic media, the voltage difference between the element and the media will be minimized and damage from a voltage discharge will be prevented.
One problem with the circuit disclosed by Jove et al., is that the circuit requires a two-sided power supply. In the preferred embodiment disclosed by Jove et al., the voltage of the MR element is maintained at the circuit ground potential (approximately zero volts), and a bias current is maintained through the MR element. Accordingly, one terminal of the MR element must be maintained above the ground voltage and the other terminal must be maintained below ground voltage. A two-sided power supply requires additional power, is more complex, and costs more than a single sided power supply.
Another technique used in the prior art to protect the magnetic surface and the MR element is to utilize a biased spindle configuration. In a biased spindle configuration, the rotating magnetic media is DC biased to some voltage away from the circuit common. Using this type of configuration, the protective circuit disclosed by Jove et al. can be operated with a single-sided power supply, with the spindle biased at a voltage level between the circuit Common and the supply voltage provided by the single-sided power supply. However, biasing the spindle at a voltage away from the circuit common is not desirable because the spindle must be electrically insulated from the rest of the drive to prevent a short-circuit.
Another problem which has proved challenging in the prior art is coupling the preamplifier output signal to other circuitry. Since the MR element must be DC biased, there is typically a DC bias in the output signal. Of course, a common method of removing a DC component from a signal is to use coupling capacitors. However, coupling capacitors are large and require significant space on a circuit board. In addition, disc drives often employ many MR elements and preamplifiers, and therefore require many coupling capacitors.
This problem was addressed by Belk in U.S. Pat. No. 4,833,559. Belk disclosed multiplexing plural MR elements into a single off chip capacitor. By multiplexing the MR elements, the number of capacitors required is minimized, and the component count is reduced. In addition, all the multiplexing transistors can be included on a single integrated circuit.
Another patent to Jove et al., U.S. Pat. No. 5,103,353, discloses an MR amplifier circuit having a first feedback loop which provides short circuit protection and a second feedback loop which minimizes the differential DC component in the output signal. The first feedback loop is similar to the feedback loop disclosed in U.S. Pat. No. 4,879,610. The second feedback loop equalizes the DC and low frequency components of two currents flowing through the collectors of two transistors. The output signal of the amplifier is derived from these two currents, and therefore, the differential DC and low frequency components of the output signal is minimized.
Although the differential DC and low frequency components are minimized, the common mode DC and low frequency components of the output signal are affected by the operation of the first feedback loop. Accordingly, coupling capacitors are still required to couple the amplifier output to other amplification circuitry that has an input stage with a different DC reference point. In addition, in a disc drive configuration not having a biased spindle, the preferred embodiment of this circuit requires a two-sided power supply to maintain the voltage of the MR element at the same voltage as the magnetic media.